The present invention relates to a cathode structure for cathode-ray-tubes and particularly to such a structure for providing high current density emission.
The performance of cathode-ray-tubes is often limited by the current density of the electron beam. In the case of the typical measurement CRT used in oscilloscopes, cathode emission limits the ability of the instrument to utilize the bandpass available. Although an electron beam can be deflected rapidly enough for viewing very "fast" events, the lack of beam current may render the resulting trace so dim that it cannot be seen. In display CRTs such as used for computer terminals, work stations and the like, the information density and resolution are limited since spot size on the CRT screen is governed by electron beam current density.
Simply running a conventional CRT cathode at a higher temperature, in order to permit increased cathode loading, drastically reduces the life span of the cathode. A higher beam current can be obtained with a "dispenser" cathode, e.g. comprising a sintered material such as tungsten impregnated with an electron emissive material. However, construction of these cathodes presents various problems. A dispenser cathode with a potted heater might produce enough heat, but such a structure is difficult and expensive to manufacture in small sizes for a CRT. When utilizing a "slip-in" or insertable heater, the structures heretofore proposed have been unable to cope with heater burn-out problems inasmuch as the heater must be operated at an excessively high temperature in order to heat the cathode to the necessary level. Prior art structures have employed re-entrant tubular metal structures having electron dispensers disposed at the end of a thin metal tube within which the heater element is located. Unfortunately, much of the heat provided is radiated or conducted away down the metal support tube rather than being effective in raising the temperature of the cathode emitter. Moreover, the prior structures have been mechanically unstable resulting in unpredictable electronic properties.